Optical coherence tomography (OCT) using the multiple-wavelength optical interference is a method for obtaining a high-resolution tomographic image of a sample (particularly, the fundus). Hereinafter, an apparatus for capturing a tomographic image by using OCT is referred to as OCT apparatus.
In recent years, OCT apparatuses for ophthalmology have attempted to acquire a functional OCT image for imaging the optical characteristics and movement of the fundus tissue in addition to an ordinary OCT image for imaging the shape of the fundus tissue. A polarization OCT apparatus, one type of the above-mentioned functional OCT apparatus, performs imaging by using polarization parameters (retardation and orientation) which are optical characteristics of the fundus tissue. By using polarization parameters, the polarization OCT apparatus can form a polarization OCT image and make a distinction or perform segmentation of the fundus tissue. The polarization OCT apparatus uses a circularly-polarized beam or polarization-modulated beam as a measuring beam for observing a sample, and splits an interference beam into two orthogonal linearly-polarized beams.
A technique for acquiring a high-resolution polarization OCT image by using a polarization OCT apparatus having two spectroscopes and a wide-wavelength light source is discussed in Non Patent Literature 1. This technique renders a tomographic image in which the retinal pigment epithelium layer is distinguished from the fundus tissue by using acquired polarization parameters. It is known that depolarization (randomization of polarization) takes place in the retinal pigment epithelium layer. The polarization OCT apparatus uses two different diffraction gratings for two interference beams having different polarization directions (orthogonal linearly-polarized beams).
A polarization OCT apparatus for detecting two interference beams by inputting two interference beams having different polarization directions to one spectroscope is discussed in Non Patent Literature 2. Thus, the polarization OCT apparatus can be downsized and control procedure can be simplified.
Generally, the diffraction efficiency of a diffraction grating, one type of spectroscopes, depends on the polarization direction. Therefore, beams having different polarization directions differ in spectral characteristics at a diffraction grating (incidence beam polarization directions with respect to the diffraction grating), resulting in different sensitivities of the diffraction grating. Thus, the accuracy in polarization parameter calculation is affected.